The development of lasers for medical use has been driven by the desire to create tissue effects that are beneficial for medical treatment. The tissue effects sought generally comprise cutting, coagulation, hemostasis, and teeth; vascular tissue such as liver and other internal organs, and tumors; fibrous tissue such as muscle and tendon, and non-fibrous tissue such as brain, marrow, and the like.
Medical lasers have been developed on the underlying physical principle that the various constituents of tissue have different absorption characteristics for various wavelengths of light. It follows that a given tissue can be analyzed to determine its primary constituents, the spectral absorption of the constituents can be measured, and a laser having a wavelength that is absorbed in the given tissue can be designed and used for treating the tissue. (This approach presumes a CW or quasi-CW laser output.) For example, water is a primary constituent of many tissue types, and the CO.sub.2 laser produces a long infrared wavelength that is absorbed very well in water. As a result, the CO.sub.2 laser is very effective in cutting and ablating many tissue types. Likewise, argon lasers and frequency doubled Nd:YAG lasers produce a green wavelength that is well absorbed in hemoglobin, resulting in good surface cutting and hemostasis of bleeding tissue surfaces. On the other hand, Nd:YAG lasers produce a near infrared wavelength that is not well absorbed in water or hemoglobin or other tissue constituents; as a result, the Nd:YAG laser beam penetrates tissue and produces desirable tissue effects such as deep coagulation that other lasers cannot equal.
There are several important disadvantages to the spectral absorption conceptual framework of lasers and tissue effects. First of all, coagulation of tissue is generally desirable not only at the surface of the tissue, but also deep beneath the surface. Laser wavelength that are well absorbed by tissue do not penetrate the tissue and cannot produce deep tissue coagulation, or any other deep tissue effects. As a result, laser light generated by CO.sub.2 lasers, Ho:YAG, or frequency doubled Nd:YAG lasers are notably inadequate in tissue coagulation.
Furthermore, due to the fact that each laser wavelength is generally optimized for a particular tissue type and tissue effect, many different lasers are required to carry out the full range of procedures that lasers are capable of performing. Considering that each laser can cost as much as $150,000, it is clear that a medical institution cannot afford to acquire all of the lasers that are necessary. Although some lasers are capable of generating plural wavelengths, these devices still cannot achieve all the tissue effects that are desired. Moreover, multi-wavelength and tunable wavelength lasers are complex, very expensive, and limited in power output. Thus it is clear that spectral absorption cannot be the primary criterion of laser design for medical use.
It has also been observed in the prior art that pulsed lasers create tissue effects that would not be predicted on the basis of spectral absorption alone. For example, brief laser pulses of relatively high energy can deposit sufficient energy in a small target spot to create a localized crater, regardless of the type of tissue which forms the target and irrespective of the spectral absorption of the laser wavelength by the tissue. This effect is capable of being used as an efficient means of ablating or cutting tissue, and is applicable to virtually any form of tissue. For example, pulsed lasers of relatively low power have been introduced recently for use in dental procedures, and their use in cutting and fusing tooth and bone tissue has been reported.
It has been theorized that pulsed lasers deposit sufficient energy in a brief time at the tissue target to heat the target surface and create a localized plasma. The plasma is generally opaque to light, so that almost all the laser energy is absorbed in the plasma at the surface of the target tissue. The disadvantage of this approach is that the brief pulses of laser energy cannot penetrate to any significant depth in tissue to achieve coagulation, hemostasis, or the like, and this drawback is similar to the drawback noted above; e.g., for CO.sub.2 lasers. Also, different tissue types require different instantaneous power densities to generate the plasma effect.
Therefore, despite the advertising claims of manufacturers and the expectations of users, there is no laser available in the prior art that can treat a wide range of tissue types and produce all the tissue effects required to serve all the treatment modalities of modern medical practice.